The semiconductor industry is increasingly being driven to decrease the size of semiconductor devices located on integrated circuits (ICs). For example, miniaturization is needed to accommodate the increasing density of ICs necessary for today's semiconductor products. Increased packing density and device size reduction has forced semiconductor device structures such as transistors to be located ever closer to one another.
As semiconductor device components become located closer together, the problem of so-called Joule heating becomes more pressing. In general, bulk flow of electrons within conventional semiconductor devices generates heat that must be dissipated. The problem of Joule heating is limiting the ability of semiconductor manufacturers to satisfy the demand for even smaller, more compact devices. Manufacturing smaller devices using known charge diffusion technologies results in increased Joule heating.
One potential solution to the Joule heating problem is to utilize the spin states of electrons rather than the charge of electrons. In addition to having a charge, electrons also have a well defined spin. “Spin” is a property of an electron that is generally related to the angular momentum of an electron about an axis within the electron. An electron has to spin states—spin up (+½) and spin down (−½). These discernable spin states can be flipped or toggled for purposes of identifying logic “0” and “1” values.
The amount of energy required to alter the electron spin may be less than the amount of energy needed for bulk charge movement (as is done in traditional semiconductor devices). For this reason, spin-based devices may offer a promising modality for very small semiconductor-based devices and provide the potential for faster logic devices, such as field-effect transistors (FETs), and may consume less power and generate less heat.
One significant challenge to the realization of spin-based FETs is the ability to electronically inject spin-polarized charge carriers (e.g., electrons) into a suitable substrate (e.g., single crystalline silicon) or semiconductor channel at room or ambient temperature. Spin-polarized refers to the state in which all or substantially all of the electrons are initialized to a given spin state (e.g., spin “up” or spin “down” state).
One known manner of initializing or polarizing electrons to have a certain spin state is based on passing the electrons or holes through ferromagnetic materials (which are metals) that have been magnetized then into semiconductor materials. More particularly, in certain known spin devices, magnetic forces spin polarize electrons as they pass through a ferromagnetic material. The spin-polarized electrons pass from a ferromagnetic material into a semiconductor-based material. Unfortunately, efficient spin injection using these types of structures may not be achievable due to the conductivity mismatch between the ferromagnetic material and the semiconductor-based material. More particularly, these effects cause electrons that were spin-polarized in the ferromagnetic material to randomize and assume various spin states when they are injected into the semiconductor material. This randomization negates or reduces the spin polarization that was achieved using ferromagnetic material, thereby making it difficult or impossible to achieve a common and detectable spin state.
Another known manner of initializing spin of electrons is to inject spin from a dilute magnetic semiconductor that serves to align spin in the presence of a magnetic field. Such devices may operate well at low temperatures but are not suitable at room temperature due to the magnetic semiconductor materials losing their spin-aligning capabilities at room temperatures.
Yet another known manner of initializing electrons to have a certain spin is based on quantum mechanical tunneling and use of an intermediate layer of silicon dioxide. Tunnel injection is, however, associated with high resistance, which is detrimental to FET operations.
A further known spin initialization method relies on optical polarization of electrons. This technique, however, has proved difficult, and it is generally believed to be incompatible or difficult to effectively implement with most microelectronic devices.
Other aspects of known spin initialization or injection devices and spin-based transistors are described in “Electrical Spin Injection and Transport in Semiconductor Spintronic Devices” by Jonker et al., “Spintronics: Fundamentals and applications,” by Zutic et al., and “Perpendicular Hot Electron Spin-Valve Effect in a New Magnetic Field Sensor: The Spin-Valve Transistor” by D. J. Monsma et al., and U.S. Publication No. 2004/0178460 A1 by Lee et al., the contents of all of which are incorporated by reference as thought set forth in full.
Lee et al., for example, describe a spin injection device and a FET having a ferromagnetic-semiconductor-ferromagnetic structure. A spin-polarized carrier is injected into the channel region from the ferromagnetic source and detected from the ferromagnetic drain. A device having such a structural configuration, however, may suffer from spin randomization, as discussed above.
As a further example, Monsma et al. describe a spin-valve transistor having a particular bipolar junction transistor (BJT) configuration (emitter, base and collector). Monsma et al. describe a BJT structure having a layered Co/Cu base between an emitter and a collector that are formed of the same material (silicon). Monsma et al. describe preparing a transistor using direct bonding, which involves forming connections by spontaneous adhesion, rather than more widely used fabrication methods of thin film deposition.
The transistor and structure described by Monsma et al., however, may not be desirable for a number of reasons. The device structure described by Monsma et al. presents significant operational and manufacturing challenges due to direct bonding fabrication, which can be particularly difficult when dealing with thin or fine scale layers or materials. Moreover, the structure described by Monsma et al. is not based on initializing and detecting a particular spin polarization. For example, Monsma et al. explain that the thickness of individual Co or Cu layers is much smaller than the spin-flip diffusion length and, therefore, spin up and spin down electrodes carry current in parallel. Consequently, a common or dominant spin state cannot be readily identified.
A further disadvantage of the device structure described by Monsma et al. is that direct bonding may result in the introduction of defects into the Co/Cu material, resulting in scattering of electrons and disruption of electron transport. Moreover, the device structure described by Monsma et al. involves modulating magnetization of the Co/Cu material, which may present issues of slow switching times. Further difficulties may arise in forming contacts on the device due to the fact that the active device is situated in between two full thickness (˜0.5 mm) Si wafers and access to the device may be difficult.
There thus is a need for spin injection devices, spin FETs that are capable of efficiently injecting spin-polarized electrons into a substrate, such as a single crystalline silicon material or substrate. Such devices, FETs and methods should be able to spin-polarize a substantial number of electrons to a particular spin state without spin alignment randomization associated with interfacial effects between ferromagnetic and semiconductor materials. Such spin injection devices should also have low resistance, which is an important figure of merit for the overall FET performance, particularly in terms of the power consumed by the device. In addition, such spin injection devices, FETs and methods should be capable of fabrication using accepted fabrication systems techniques and be amenable to incorporation into current and contemplated microelectronic devices. Further, such devices and FETs should be operable at room or ambient temperatures so that they can be used in various commercial devices and applications without environmental limitations. It would also be desirable to have spin FETs that can serve as an alternative to known silicon CMOS devices that are based on charge diffusion.